1. Field of the Invention
The invention relates to a welding head for a looping machine.
2. Background Art
Welding heads of the generic type come in various designs. Parts common to these welding heads are a strap guide passing the looping strap through the welding head, a strap fixing element holding the lapping strap sections which are to be welded together, a welding set melting the strap sections to be welded together, and a pressing arrangement joining the melted strap sections which are to be welded together. Examples of welding heads of the species are illustrated in German utility models DE 297 16 896 U1, DE 297 16 897 U1 and DE 297 16 898 U1.
When thin unstable stacks of, for instance, printed matters are looped, loading the product stack is possible only to a very restricted extentxe2x80x94if possible at allxe2x80x94because otherwise the stack would wave or fold. In this regard, the looping machine can operate only at a very restricted loop tensioning force. Since, owing to its instability, the product to be looped hardly offers any resistance to the tensioning of the strap, there is no stretching of the looping strap itself. This means that no contraction forces are inherently available in the strap, as a result of which the lapping and welded strap sections in the welding head will not come out of the welding head by themselves by the portion of the strap where the loop is closed. The noose of the strap possesses too low an elasticity of its own to enable the closed portion of the strap to come out of the welding head.
To solve these problems, actuation of the looping strap by compressed air has been proposed, the strap thus being blown out of the welding head. The air jets needed to this end are disposed beside the welding head or incorporated therein, which is accompanied with considerable constructional requirements.
The compressed air supply means necessary in this design causes additional expenditure for the supply infrastructure of a looping machine equipped with such a welding head. Very often, compressed air supply is not even available.
Proceeding from these problems, it is an object of the invention to embody a welding head for a looping,machine which can be employed universally and without compressed air supply means and which reliably enables the welded strap sections to come out of the welding head.
This object is attained by an ejector of mechanical operation which is integrated into the welding head, lifting the welded looping strap out of the welding head.
Because of the mechanical characteristics of the ejector, compressed air supply means can be omitted. This also implies reduced noise pollution during operation of the machine, because a mechanical ejector can work by far more quietly than hissing compressed air jets. Finally, from aspects of construction and control technique, a mechanical ejector is by far more easily to be put into practice and works more reliably.
Preferred embodiments of the looping machine and details of an exemplary embodiment of the invention will become apparent from the ensuing description, taken in conjunction with the annexed drawings.